An image forming apparatus has been known such as a printer and a multi-function peripheral (MFP) which prints an original image such as a text, a figure, and a picture in a recording medium (for example, a sheet) as a visible image. Further, an image forming apparatus having a function of printing a specific image at a predetermined position in the recording medium in addition to a function of printing the original image has been proposed. The image forming apparatus forms the specific image in the recording medium as an image unrecognizable for human eyes for maintaining security. The specific image unrecognizable for human eyes, for example, can be visualized by a method of irradiating the image with a light source having a specific wavelength.
Furthermore, in the image forming apparatus, there is proposed a technology in which an image is printed in the recording medium by a decolorable color material and the image of the decolorable color material is decolorized to reuse the recording medium. For example, the image forming apparatus performs an image forming process in which a toner image is formed in the recording medium using toner as the decolorable color material in an electrophotographic manner. Furthermore, the image forming apparatus performs a decolorization process in which the decolorable toner image is decolorized in a state where the decolorable toner image of the recording medium is heated at a temperature equal to or more than a decolorization temperature. The image forming apparatus repeatedly performs the image forming process and the decolorization process in order to reuse the recording medium.
Even when the decolorable toner image is decolorized by the image forming apparatus, the transparent decolorable toner itself is left in the decolorable toner. Therefore, for example, when the specific image is printed at a predetermined position (the same position) of the recording medium, the decolorable toner image of the specific image is overlapped on the decolorable toner left which is previously decolorized and left in the recording medium. Even when the overlapped decolorable toner image is heated at a temperature equal to or more than the decolorization temperature and decolorized, the decolorable toner image is stacked and left in the recording medium as a toner layer. In a case where the number of reused times (the number of repetitions of the image forming process) of the recording medium is increased, a thickness of the decolorable toner layer is increased. Basically, the decolorization process of the image is to make the image hardly recognized by humans, but there may be a phenomenon that the decolorized image floats up by the thickness of the decolorable toner layer and is visualized.